The present invention relates to mold facecoats and corecoats for use in the fabrication of molds for casting reactive metals, particularly complex shapes thereof.
Melting and investment casting of reactive metals, such as titanium or titanium alloy, is made difficult due to the reactive metal's affinity for elements such as oxygen, nitrogen and carbon. At elevated temperatures, the reactive metals tend to react with almost any type of containment incorporating such elements. For example, at elevated temperatures during investment casting, solidification and cooling, a Ti-6Al-4V alloy reacts with oxygen and/or most oxide ceramics to form an oxygen enriched surface layer. This surface layer, commonly referred to as an "alpha-case" or a "coarse basket weave," can be brittle and is therefore detrimental to the mechanical properties of the casting and thus must be removed.
Typically, removal of oxygen or interstitial element enriched surface material is accomplished by mechanical or chemical means such as chemical milling in an acid bath. This process, however, is not straightforward, since the thickness of the alpha-case on an as-cast component varies for each section of the component depending on the thickness and solidification rate of the section. On the other hand, chemical milling removes surface material at an essentially uniform rate regardless of the section's thickness. Consequently, numerous iterations may be necessary to determine the proper wax pattern die size which must be utilized to generate a chemically milled component having the required finished product dimensions.
In investment casting, mold/metal reactivity traditionally has been reduced or eliminated by using facecoat or corecoat materials such as carbon or graphite, high temperature oxides, refractory metals, halide salts or the reactive metals themselves. These traditional containment methods usually are expensive, complex or even potentially hazardous such as when radioactive materials such as ThO.sub.2 are used as the facecoat or corecoat material. In addition, these traditional facecoat and corecoat materials present the following technical limitations: (1) they are often difficult to apply; (2) they often require controlled atmosphere firing and pre-heating; (3) even with these materials there can still be a substantial risk of contamination from mold materials; and (4) the castings produced generally exhibit a substantial section thickness dependent reaction layer which must be removed, thereby causing difficulty in determining the as-cast part size necessary to produce the finished part.
For a number of years, yttria (Y.sub.2 O.sub.3) has been investigated as a possible mold facecoat material because of its low reactivity with respect to titanium. To make application of yttria economical, investigators have tried yttria-based slurries. Heretofore, however, investigators have been unsuccessful in using yttria-based slurries as mold facecoat materials in the fabrication of molds for casting reactive metals.
For example in 1976, Schuyler et al. reported the results of tests using fine particle yttria dispersed in colloidal potassium silicate solution to which coarse yttria has been added as a mold facecoat material. D. R. Schuyler, et al., "Development of Titanium Alloy Casting Technology," AFML-TR-76-80, August 1976, pp. 275-279. The molds made with this facecoat material were not satisfactory. Schuyler et al. reported that "the facecoat was not as smooth as normal for the standard foundry system. Pores and pits were present, and the stucco showed through in many places." Schuyler et al. also tried a slurry containing yttria, titania and colloidal silica. Schuyler et al. found that with this system the facecoat surface was even more highly pitted.
Further unsuccessful attempts to use an yttria-based slurry as a mold facecoat material were reported by Calvert in 1981. E. D. Calvert, "An Investment Mold for Titanium Casting," Bureau of Mines, RI8541, pp. 5-7, 1981. Calvert reported that mold facecoat compositions comprising yttria powder and aqueous colloidal silica binder resulted in slurries which exhibited rapid and premature gellation and also resulted in mold surfaces which exhibited a tendency to crack and spall during mold firing. Similar results were obtained with yttria-based slurries comprising yttria powder and a zirconium acetate binder. Calvert also tried adding H.sub.2 SO.sub.4 to the yttria-based slurry but this caused porosity in the resulting titanium investment casting.